The role of vasopressin in the control of renal water excretion has long been known. Evidence developed in recent years suggests that vasopressin may also be important in the physiological control of the cardiovascular system, and these has been considerable interest in the possible participation of vasopressin in the pathogenesis of some forms of clinical and experimental hypertension. It has been the long-term objective of this research program to identify and study the factors which determine the plasma concentration of vasopressin. The present application is concerned with the role of brain neurotransmitters and neuromodulators involved in the stimulation of vasopressin release by increased plasma osmolality and by decreased blood volume and blood pressure. Brain prostanoids appear to play an important role in the osmotic and volume-pressure control of vasopressin release, by modulating the activity of the relevant neurotransmitters or by direct actions on the vasopressinergic neurosecretory cells. It is proposed to test this hypothesis and to identify the brain neurotransmitters involved in this control and the sites in the brain where these actions occur.